configs: Use absolute import paths

Use absoluate import paths to be Python 3 compatible. This also
imports absolute_import from __future__ to ensure that Python 2.7
behaves the same way as Python 3.

Change-Id: Ica06ed95814e9cd3e768b3e1785075e36f6e56d0
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/16708
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>

configs/learning_gem5/part2/hello_goodbye.py

@@ -37,6 +37,7 @@ IMPORTANT: If you modify this file, it's likely that the Learning gem5 book
 """
 
 from __future__ import print_function
+from __future__ import absolute_import
 
 # import the m5 (gem5) library created when gem5 is built
 import m5